This invention relates to a method for treating seaweed, more specifically to a method for stripping off the outer skin (exodermis) of the seaweed for preparing seaweed-powder or algin.
Although various seaweeds, such a Laminaria or Undaria pinnatifida, have provided sources of useful alkali foods for human consumption since ancient time, in their natural forms these plants themselves are not necessarily suitable as foods. For example, seaweed products commonly marketed are prepared by simply drying the natural seaweed. As these products have the seaweed skin intact, they have a bad appearance, being dull in tone and giving the impression of being unclean. Furthermore, they are difficult to digest. Seaweeds are also boiled to produce soup base and stock concentrates. However, naturally available seaweed is disadvantageous in producing such concentrates since the skins thereof impair the quality as well as digestibility of the soups and stocks prepared from the concentrates.
Another applications of seaweed is the preparation of algin therefrom.
Alginic acid, a kind of mannuronan belonging to the polyuronide family, is a linear molecule composed of D-mannuroic acid having .beta.-1,4 bonds therein. This acid is an important polysaccharide which is contained in seaweeds, particularly in phaephyta. Algin is the generic name of various salts of alginic acid and the sodium salt, ammonium salt and propyleneglycohol ester of alginic acid are of particular commercial significance.
Although algin has a variety of applications, because of its excellent emulsifying, stabilizing and water-absorbing actions, such as gelling agent, water-soluble ointment, lubricant, paper-sizing agent, finishing agent for leather or fabrics, and even pasting agent and childbirth-assisting agent, it is extremely troublesome to handle because of its high viscosity (1000 centipoises in 1% solution, 8000 centipoises in 2% solution). The conventional method for recovering algin from seaweeds is thus disadvantageous in that it comprises complicated processing steps and requires a long time thus resulting in high cost which has prevented the wide usage of said method.
The following is an example of a complicated method conventionally practised.
Seaweed such as Laminaria, after being washed with water, is treated for several hours, for example, with 0.05N sulfuric acid to induce swelling, and then is treated with caustic soda or soda ash followed by stirring of the mixture for over 5 hours at a pH of approx. 9.5. The solution thus obtained is highly viscous, and is diluted with water for lowering the viscosity to prepare a dilute solution of algin, for example, of 0.85 g/1. (The dilution step is indispensable in the conventional method.) A filtration step follows. However, the solution is difficult to filter per se. Filtration is carried out after the insoluble residue composed of such as the outer skins of the seaweed is removed from the solution with wire-netting using a filtration-assisting agent such as diatomaceous earth. The fltrate is thereafter neutralized with sulfuric acid to produce alginic acid, which is taken up by flotation method for preparing a solution, for example, of 2%. The solution is further neutralized with alkali, the resulting gel being decolorized and dehydrated over a long period of time. The final product is obtained by repeating these steps several times.
As explained above, the conventional method for recovering algin from seaweed is disadvantageous since it requires a number of troublesome steps due to the high viscosity of alginic acid and its salts, and particularly requires a step for removing impurities such as the outer skins from the seaweed or a step for decolorizing the highly viscous extract product of the seaweed. An attempt to avoid the high viscosity by heating fails since it causes considerable change in the chemical structure of the compounds. For example, heat treatment for an hour at 95.degree. C gives rise to 37% lowering in viscosity, and that for an hour even at 80.degree. C give rise to 8% lowering in viscocity, indicating substantial change in chemical structure.